1. Field of the Invention
This invention relates generally to implantable devices for interventional therapeutic treatment or vascular surgery, and more particularly concerns a coated superelastic stent formed from a stranded micro-cable with enhanced radiopacity.
2. Description of Related Art
The art and science of interventional therapy and surgery has continually progressed towards treatment of internal defects and diseases by use of ever smaller incisions or access through the vasculature or body openings in order to reduce the trauma to tissue surrounding the treatment site. One important aspect of such treatments involves the use of catheters to place therapeutic devices at a treatment site by access through the vasculature. Examples of such procedures include transluminal angioplasty, placement of stents to reinforce the walls of a blood vessel or the like and the use of vasoocclusive devices to treat defects in the vasculature. There is a constant drive by those practicing in the art to develop new and more capable systems for such applications. When coupled with developments in biological treatment capabilities, there is an expanding need for technologies that enhance the performance of interventional therapeutic devices and systems.
One specific field of interventional therapy that has been able to advantageously use recent developments in technology is the treatment of neurovascular defects. More specifically, as smaller and more capable structures and materials have been developed, treatment ofvascular defects in the human brain which were previously untreatable or represented unacceptable risks via conventional surgery have become amenable to treatment. One type of nonsurgical therapy that has become advantageous for the treatment of defects in the neurovasculature has been the placement by way of a catheter ofvasoocclusive devices in a damaged portion of a vein or artery.
Vasoocclusion devices are therapeutic devices that are placed within the vasculature of the human body, typically via a catheter, either to block the flow of blood through a vessel making up that portion of the vasculature through the formation of an embolus or to form such an embolus within an aneurysm stemming from the vessel. The vasoocclusive devices can take a variety of configurations, and are generally formed of one or more elements that are larger in the deployed configuration than when they are within the delivery catheter prior to placement. One widely used vasoocclusive device is a helical wire coil having a deployed configuration which may be dimensioned to engage the walls of the vessels. One anatomically shaped vasoocclusive device that forms itself into a shape of an anatomical cavity such as an aneurysm and is made of a preformed strand of flexible material that can be a nickel-titanium alloy is known from U.S. Pat. No. 5,645,558, which is specifically incorporated by reference herein.
The delivery of such vasoocclusive devices can be accomplished by a variety of means, including via a catheter in which the device is pushed through the catheter by a pusher to deploy the device. The vasoocclusive devices, which can have a primary shape of a coil of wire that is then formed into a more complex secondary shape, can be produced in such a way that they will pass through the lumen of a catheter in a linear shape and take on a complex shape as originally formed after being deployed into the area of interest, such as an aneurysm. A variety of detachment mechanisms to release the device from a pusher have been developed and are known in the art.
For treatment of areas of the small diameter vasculature such as a small artery or vein in the brain, for example, and for treatment of aneurysms and the like, micro-coils formed of very small diameter wire are used in order to restrict, reinforce, or to occlude such small diameter areas of the vasculature. A variety of materials have been suggested for use in such micro-coils, including nickel-titanium alloys, copper, stainless steel, platinum, tungsten, various plastics or the like, each of which offers certain benefits in various applications. Nickel-titanium alloys are particularly advantageous for the fabrication of such micro coils, in that they can have super-elastic or shape memory properties, and thus can be manufactured to easily fit into a linear portion of a catheter, but attain their originally formed, more complex shape when deployed. Although various materials are more or less kink resistant when nickel-titanium alloys are dimensioned into wire smaller than approximately 0.010 inches in diameter, they can have low yield strength and can kink more easily, thus severely limiting the applications for such finely drawn wire in the fabrication of vasoocclusive devices. As a further limitation to such applications, nickel-titanium alloys are also not radiopaque in small diameters, and a single nickel-titanium wire would need to be approximately 0.012 inches in diameter to be even slightly radiopaque. However, such a thickness of a single nickel-titanium wire would unfortunately also be relatively stiff and possibly traumatic to the placement site, particularly if used for treatment of delicate and already damaged areas of the small diameter vasculature such as an aneurysm in an artery or vein in the brain, for example.
One conventional guidewire for use in a catheter is known that is made of a high elasticity nickel-titanium alloy, and is useful for accessing peripheral or soft tissue targets. The distal tip of the guidewire is provided with a radiopaque flexible coil tip, and a radiopaque end cap is attached to the guidewire by a radiopaque ribbon. Such a construction is complex to manufacture, fragile and can potentially break off during use with undesirable results. A stretch resistant vasoocclusive coil is also known that can be made of a primary helically wound coil of platinum wire, with a stretch-resisting wire attached within the primary coil between two end caps. Unfortunately, such a construction is relatively difficult to fabricate and also fragile, allowing for the possibility of the fracture of the central radiopaque wire, the coil, the welds or some combination of them, and it can also potentially break off during use. Also, such a construction has a complex and nonlinear bending characteristic, dependent on the spacing of the coils and central wire and the radius of the bend of the coil.
Stents are typically implanted within a vessel in a contracted state and expanded when in place in the vessel in order to maintain patency of the vessel, and such stents are typically implanted by mounting the stent on a balloon portion of a balloon catheter, positioning the stent in a body lumen, and expanding the stent to an expanded state by inflating the balloon. The balloon is then deflated and removed, leaving the stent in place. However, the placement, inflation and deflation of a balloon catheter is a complicated procedure that involves additional risks beyond the implantation of the stent, so that it would be desirable to provide a stent that can be more simply placed in the site to be treated in a compressed state, and expanded to leave the stent in place.
Stents also commonly have a metallic structure to provide the strength required to function as a stent, but typically do not provide for the delivery of localized therapeutic pharmacological treatment of a vessel at the location being treated with the stent. Stents formed of polymeric materials capable of absorbing and releasing therapeutic agents may not provide adequate structural and mechanical requirements for a stent, especially when the polymeric materials are loaded with a drug, since drug loading of a polymeric material can significantly affect the structural and mechanical properties of the polymeric material. Since it is frequently desirable to be able to provide localized therapeutic pharmacological treatment of a vessel at the location being treated with the stent, it would be desirable to combine such polymeric materials with a stent structure to provide the stent with the capability of absorbing and delivering therapeutic drugs or other agents at a specific site in the vasculature to be treated.
Conventional forms of stents are known that have a covering or outer layers of collagen, that can be used for enhancing biocompatability and for drug delivery. One known tubular metal stent, for example, is combined with a covering sleeve of collagen in order to increase the biocompatibility of the stent upon implantation. The collagen sleeve may be collagen per se or a collagen carried on a support of Dacron or a similar material. Another three-layer type of vascular prosthesis has two outer layers formed of collagen, and a middle layer made from inert fibers such as synthetic fibers. Another tubular reinforcing structure for use as a cardiovascular graft is made of collagenous tissue with a reinforcing fibrous structure surrounding the lumen. One drug delivery collagen-impregnated synthetic vascular graft is also known, formed from a porous synthetic material having collagen such that the graft substrate cross-links the collagen in order to render the substrate blood-tight. A matrix is also provided in another biodegradable drug delivery vascular stent that is made from collagen or other connective proteins or natural materials that can be saturated with drugs. However, none of these types of stents provide for a coated, superelastic shape memory stent that can be delivered and released at the site in the vasculature to be treated in a compressed state, and expanded to leave the stent in place without the need for placement with a balloon catheter.
From the above, it can be seen that vasoocclusive devices and stents provide important improvements in the treatment of the vasculature. However, it would be desirable to provide a structural element that used to form a coated stent, that offers the advantages of a shape memory alloy such as a nickel-titanium alloy, and that incorporates radiopaque material in a stable configuration that is not subject to breaking during use of the device, so that the device can be visualized under fluoroscopy. The present invention meets these and other needs.